In recent years, the development of fuel cell vehicles which travel using electric power supplied from a fuel cell system has been promoted as a countermeasure against the exhaustion of oil resources and global warming. Fuel cell systems installed in such fuel cell vehicles are required to maintain power generation performance for as long a time period as possible without maintenance.
There are various factors that degrade the power generation performance of a fuel cell system. One known phenomenon is for a catalyst to be eluted from a fuel electrode in a cell. If the cell is kept at a high voltage state for a long period of time, part of a platinum catalyst supported on the fuel electrode in the cell melts and becomes unable to function as a catalyst for the fuel electrode. If the catalyst is eluted, the I-V characteristics of the cell will be degraded and the maximum value of electric power that can be generated will thus be decreased.
It has been known that the higher the power-generation voltage of the cell is and the longer the high power-generation voltage state is maintained, the more likely the above catalyst elution is to occur. Thus, in a fuel cell system described in Patent Document 1 below, high-potential avoidance control is performed such that the voltage of a cell is decreased by temporarily increasing power consumption of an auxiliary machine so as not to allow the voltage to exceed a preset upper limit value of the voltage. By performing such high-potential avoidance control, the voltage of the cell is maintained so that it is equal to or lower than the upper limit value, to thereby prevent catalyst elution and achieve a prolonged lifetime for a fuel cell system.